To improve their temperature and/or abrasion resistance, turbine blades or vanes, such as guide vanes and rotor blades which are intended for gas turbines, are coated with suitable metals, metal alloys or ceramics. The coating is done by means of a spray coating apparatus in which the turbine blade or vane is spray-coated. Examples of spray-coating processes include atmospheric plasma spraying (APS) and high-velocity oxyfuel spraying (HVOF) (cf. Ullmanns Encyclopedia of Industrial Chemistry, 2003, Vol 21, pages 573 and 575).
In the context of turbine blades or vanes, a distinction is drawn between guide vanes and rotor blades. Both have an airfoil which is exposed to the hot gas and at one end merges into a root body which serves to secure the turbine blade or vane either to a rotor (in the case of a rotor blade) or to a holder (in the case of a guide vane). At the other end of the airfoil, a guide vane additionally has a head body which, like the root body, is intended for securing to a holder. At the transition to the airfoil, root and head bodies form endplates in the form of root plates or head plates, which have a hot-gas side facing the airfoil and adjoining peripheral surfaces.
The coating described above is carried out only on those surfaces which are exposed to the hot gas, i.e. the airfoil and the hot-gas sides of the root plate and if present also head plate. The peripheral surfaces of these endplates and also the remaining parts of the root body and if present head body, according to the specification, must remain free of coating, since they have already been machined to their final dimensions. Therefore, in the spray coating apparatus the root body and if present also the head body are covered as far as possible, apart from the respective hot-gas side. However, it is virtually inevitable that coating material will also reach those parts of the peripheral surfaces of the endplates which are adjacent to the hot-gas side, i.e. that some overspray, as it is known, will occur. This requires the coating to be removed by grinding (overspray grinding) in a subsequent process step. This presents the risk of uncoated parts of the peripheral surfaces also being ground, with the result that their final dimensions change.